Plasma etch processes and apparatus are generally well known for etching materials for semiconductor device fabrication. The process begins with application of a masking material, such as photoresist, to a silicon wafer. The masking pattern protects areas of the wafer from the etch process. The wafer is then placed in a plasma reactor ("etcher") and etched. Subsequent steps are determined by the type of device being fabricated. This process is especially valuable for the definition of small geometries.
A common silicon etch process is based on fluorine. When mixtures such as CF.sub.4 -O.sub.2 are dissociated in an electrical discharge, fluorine atoms are liberated, and volatize the silicon as SiF.sub.4. Such processes are isotropic, i.e., they etch in all directions at the same rate. Anisotropic, or vertical etches in silicon are not observed when fluorine is the sole etchant.
For vertical (anisotropic) etches of silicon, the use of gas mixtures such as C.sub.2 F.sub.6 -Cl.sub.2 is known. The C.sub.2 F.sub.6 serves as a source of "recombinants", such as C.sub.3. The recombinants suppress (lateral) etching in the horizontal direction (in the plane of the wafer) by recombining with Cl atoms which have been adsorbed on the etched walls. Etching can proceed in the vertical direction (normal to the wafer) because ion bombardment from the plasma suppresses the recombination mechanism.
Submicron polysilicon ("poly") gate patterning requires minimum etch bias and vertical sidewall profiles (anisotropy). In other words, the etching process should not undercut the mask, and the poly line should not be narrower at the poly-oxide interface than it is at the mask-poly interface. FIG. 1A illustrates "acceptable" polysilicon profiles. FIG. 1B illustrates an unacceptable profile that is "re-entrant", or "negative", as would result from the aforementioned undercutting or narrowing. FIG. 1C illustrates an unacceptable profile that is notched, such as would result from a lack of sidewall protection.
Further, the selectivity of etch between poly and the underlying gate oxide (poly:oxide selectivity) must be as high as possible so as to reduce oxide loss. These two requirements, anisotropy and poly:oxide selectivity, can be fulfilled with chlorine-based dry etching processes, as opposed to fluorine-containing plasmas that tend to etch isotropically and have poor selectivity of poly:oxide. Still, even with chlorine-based processes, the selectivity of poly:oxide is compromised for profile control.
Using a typical etcher, such as the top-powered, wafer-grounded LAM 490 etcher, poly:oxide selectivities in excess of 100:1 can be achieved with low power (100 Watt) Cl.sub.2 /He plasmas, but result in severe undercutting or notching of the poly sidewall (See FIGS. 1B, 1C and 2A). On the other hand, helium-rich (greater than 50%), high power (greater than 200 Watts) processes produce vertical sidewalls at the expense of selectivity (less than 30:1) and tend to incidentally damage the gate oxide. Lateral etching occurs during the overetch cycle and not during bulk poly removal. Hence, the amount of overetch must be limited.
For purposes of the present disclosure, an etch is considered "anisotropic" if no point of the resulting polysilicon profile lies under the mask beyond 0.05 um from the mask edge at the mask-poly interface and after etching is completed, including overetch. This is illustrated in FIG. 1D.
Although etching and sidewall angle formation mechanisms are not well understood or characterized, it is widely recognized that the directionality and energy of ion bombardment from ions accelerated across the plasma sheath, and surface passivation mechanisms from redeposition of materials, play a major role in controlling anisotropy. (See, e.g., "Relation between the RF discharge parameters and plasma etch rates, selectivity and anisotropy", C. B. Zarowin, J. Vac. Sci. Technol., A, Vol.2, No. 4, October-December 1984, and "Controlled Film Formation during CCl.sub.4 Plasma Etching", S. E. Bernacki and B. B. Kosicki, J. Electrochem. Soc., Vol 131, No. 8, August, 1984.) For example, when a photoresist mask is used for patterning the poly gates, it can contribute to forming a protective film along the poly sidewall, thereby preventing lateral etching. Vertical profiles can also be obtained with oxide masks, in which case sidewall protection may result from redeposition of etched silicon-containing material, or from the gas-phase chemistry itself. In addition, the shape of the poly sidewall will depend on the poly doping level and the doping method (e.g., POCl.sub.3, implant). Heavily doped poly is more likely to etch isotropically. Non-annealed, implanted poly tends to form a notch half way along the profile (See e.g., FIG. 1C), possibly at the location of peak dopant concentration, and it terminates by a "foot" at the oxide interface where dopant concentration is minimum.
The use of Bromine (Br) containing plasmas is becoming increasingly popular for high selectivity anisotropic poly etch. Compared to chlorine, it is found to further increase the poly:oxide selectivity, and especially the poly:resist selectivity, while maintaining anisotropy with long overetch percentages (e.g., 100%). High selectivity still works against profile control, but the threshold of compromise is pushed far enough that one has to worry about other factors, such as removal of the passivation layer.
Most, if not all, plasma etcher manufacturers now offer poly etch processes using bromine. Magnetically enhanced (magnetron) etchers from Materials Research Corp., such as the MRC Aries (Trademark), and Applied Materials, such as the AMAT Precision 5000 (Trademark), use Cl.sub.2 /HBr, while those from Tegal Corp., such as the MCR (Trademark), use pure Br.sub.2. Etchers from Lam Research Corp., such as the Rainbow 4400 (Trademark), offer a process using Cl.sub.2 /HBr/He. These etchers have high selectivity and good profile control. Unfortunately, because bromine enhances sidewall passivation, it also forms a sidewall film that cannot always be stripped either in sulfur peroxide or by ashing, but requires an HF dip. An HF dip is not desirable, mostly because it etches some of the gate oxide, but also because it adds a step to the process. In such a case, the advantage of high poly:oxide selectivity offered by using Br is partly lost.
With previous methods of polysilicon ("poly") etching, using Chlorine (Cl.sub.2), there has been an inherent compromise between anisotropy and selectivity to oxide. High selectivity processes tend to be isotropic.